1. Introduction
This invention relates to a contact lens having its anterior surface coated with an evaporation barrier layer prior to or following insertion of the same into the eye. The invention also includes a solution that enables formation of the evaporation barrier layer and storage of the lens. More particularly, this invention relates to a means for preventing ocular desiccation during use of contact lenses.
2. Description of the Prior Art
It is known that insertion of a contact lens onto the eye and prolonged wearing of the lens can cause discomfort. There are many known reasons to explain the cause of the discomfort including lens fitting characteristics, lens edge design, lens movement, oxygen deprivation, ocular sensitivities, immune reaction, lid positioning, etc. However, one of the most, if not the most significant cause of discomfort, is the disruption of the protective lipid layer of the tear film over the ocular service resulting in subsequent desiccation of the ocular surface. Excessive evaporation also occurs from the front (anterior) surface of the contact lens. Ocular desiccation as a consequence of hydrogel contact lens wear was observed by Guillon et al., Ocular Desiccation Staining With Hydrogel Lenses: Tear Film and Contact Lens Factors, Ophthalmic Physiol Opt, October, 1990, 10 (4) pp. 343-350. The authors report their observation that during monitoring the response of established contact lens wearers, "ocular desiccation staining was present and associated with a rapidly destabilizing pre-lens tear film (PLTF) and a thinning lipid layer". "The initial breakup or disruption of the PLTF and corneal staining were recorded with a higher incidence in the vertical quadrants than in the horizontal quadrants. The PLTF is thinnest and most unstable at the tear-prism margin border, hence least efficacious at preventing evaporation". It was further observed by the authors that: "ocular desiccation staining may be due, at least partly, to excessive evaporation at the contact lens front surface."
An explanation similar to Guillon's was offered by Korb, Lacrimal Gland Tear Film, and Dry Eye Syndromes, edited by D. A. Sullivan, Plenum Press, New York, 1994, pp. 403-410. Korb, citing A. Tomlinson and T. H. Cedarstaff, Tear Evaporation from the Human Eye: The Effects of Contact Lens Wear, J. Br. C.L. Assoc. 5:141-150 (1982) and Guillon in Tear Film Photography and Contact Lens Wear, J. Brit. C. L. Assoc. 5:84-87 (1982), states that the lipid layer is compromised by the presence of a contact lens which leads to increased evaporation. Korb observed from his investigations that a normal lipid layer is not present with rigid lenses and that lipid layers on hydrogel lenses are variable and rarely similar to the lipid layers observed on eyes without lenses. Korb, in quoting M. J. Refojo, The tear film and contact lenses: the effect of water evaporation from the ocular surface, Simposio del Societa Optolmolgia Italiana, Rome (1984), stated that "`when a contact lens is placed in the eye, the lens alters the normal structure of the tear film and affects its rate of evaporation. These changes affect the ocular surface as well as the contact lens itself`". Korb, in summary stated that: "Both blinking and evaporation are recognized as critical factors in contact lens wear. Thus, future goals should be to increase the pre-comeal tear film thickness, and particularly to increase lipid layer thickness and quality, in order to minimize evaporation and maintain ocular surface integrity. New contact lens materials are required which would mimic the ocular surface so as to allow a pre-lens tear film which more nearly replicates the remarkably efficacious pre-ocular tear film, complete with lipid layer. This would decrease the evaporation rate and thus improve contact lens tolerance".
In a subsequent publication by Korb, entitled "The Effect of Periocular Humidity on the Tear Film Lipid Layer, Investigative Ophthalmology and Visual Science, Vol. 1995, Vol. 36, it was reported that: "Increased periocular humidity results in an increase in tear film lipid layer thickness, possibly by providing an environment that is more conducive to the spreading of meibomian lipid and its incorporation into the tear film." This conclusion was reached by placing modified swim goggles over the eyes of patients for 60 minutes and monitoring the tear film. The goggles functioned as an evaporation barrier over the eye creating a condition of high humidity in the enclosed space between the goggles and the eye. It was observed that the lipid layer increased significantly in thickness within 5 minutes of exposure to conditions of high humidity reaching a maximum increase after 15 minutes of goggle wear. This increase to a maximum lipid layer thickness was maintained at the 30 and 60 minute goggle time points. Following the goggle removal, the values declined but remained significantly elevated over the lipid layer thickness of the control contralateral eye throughout the 60 minute post-goggle period. Moderate to total relief of dry eye symptoms was reported during goggle wear and generally persisted at a reduced level for 1-3 hours following goggle removal. Thus, the high humidity contributed to the formation of an improved lipid layer over the surface of the eye, and also contributed to the decrease in comfort caused by the conditions known as dry eye. Also, in the above cited Korb publication, it was reported that goggles creating a 100% humidity environment for 30 minutes would improve the prelens lipid layer thickness in all instances.
Conclusions that can be reached from the above publications are that the lipid layer over the eye preserves the aqueous tear film. An evaporation barrier over the eye leading to an environment of increased humidity appears to enhance the tear film. Placement of the lens on an eye disrupts the lipid layer and thus leads to ocular desiccation, a decrease in the aqueous layer and discomfort associated with the wearing of a lens.